In the sequencing batch reactor process of waste water treatment, a basin or reactor is operated in a batch treatment mode involving a fill phase, a react phase, a settle phase, and a decant phase. In general, the basin is filled with waste water during the fill phase, mixed and aerated either continuously or intermittently during the react phase; the suspended solids allowed to settle during the settle phase; and the relatively clear supernatent withdrawn during the decant phase. The duration and timing of the several phases can vary in different installations and some of the phases can overlap.
The quality of the effluent withdrawn during the decant phase is markedly affected by the decanter design. While most of the suspended solids settle to the bottom during the settle phase, some solids do not settle but instead float on the surface of the basin. If the decanter draws in floating solids or pulls in lighter sludge from the lower levels in the tank during the decant phase, the effluent quality is adversely affected. Moreover, if the suspended solids enter the decanter during the fill, react and settle phases, they will be subsequently discharged with the decanted effluent during the decant phase and adversely affect the quality of the effluent.
Various different decanter designs are illustrated and described in an article entitled "Technology Evaluation of Sequencing Batch Reactors", dated September 1984, and submitted to the U.S. Environmental Protection Agency, Environmental Research Center, Cincinnati, Ohio, Contract No. 68-03-1821, by James M. Montgomery Consulting Engineers, Inc., pages 4-1 through 4-11, FIGS. 4 through 11. Some employ a fixed decanter head and some others provide a decanter head that floats so as to always withdraw from the upper layer of liquid in the basin. As pointed out in the aforementioned article, preventing entrance of solids into the decanter head and piping during the fill, react and settle phases has presented a major problem. Some decanter designs, for example as disclosed in U.S. Pat. No. 4,596,658, provide a special upwardly directed decanting orifice zone that communicates at its lower end with the water in the basin and which has an air retention zone at the upper end of the decanter orifice zone to inhibit entrance of solids into the decanter during the fill, react and settle phases. It has also been proposed as disclosed in U.S. Pat. No. 4,601,833 to provide a floating decanter and to lift the decanter entirely out of the water in the basin during the fill, react and settle phases, and to provide a trough to cover the inlet of the decanter during lifting the decanter out of the basin and lowering of the decanter back into the basin, to prevent entrance of floating debris.